In recent years, an electronic technology called “spintronics” has received attention. Until now, in an electronics area, only the “electric charge” that is one of properties of an electron is used. However, in spintronics technology, a spin that is another property of an electron is proactively used in addition to the electric charge. In particular, a “spin-current” that is a flow of spin angular momentum of the electron is an important concept. Because energy dissipation of the spin-current is small, there is a possibility that highly-efficient information transmission can be realized by using the spin-current. Accordingly, the generation, the detection, and the control of spin-current are important themes.
For example, a phenomenon in which when an electric current flows, the spin-current is generated is known. This is called “spin-Hall effect”. Further, as a converse phenomenon, a phenomenon in which when the spin-current flows, an electromotive force is generated is known. This is called “inverse spin-Hall effect”. By using the inverse spin-Hall effect, the spin-current can be detected. Further, the spin-Hall effect and the inverse spin-Hall effect significantly appears in a material (for example: Pt and Pd) with large spin orbit coupling.
In recent studies, it is shown that the spin-Seebeck effect appears in magnetic materials. The spin-Seebeck effect is a phenomenon in which when a temperature gradient is applied to the magnetic material, the spin-current is induced in a direction parallel to the temperature gradient (refer to for example, patent literature 1, non-patent literature 1, and non-patent literature 2). Namely, heat is converted into the spin current (heat to spin current conversion) by the spin-Seebeck effect. In patent literature 1, it is shown that the spin-Seebeck effect appears in an NiFe film that is a ferromagnetic metal. In non-patent literatures 1 and 2, it is shown that the spin-Seebeck effect is observed by using a magnetic insulating material such as yttrium-iron garnet (YIG, Y3Fe5O12) and a metallic film.
Further, the spin-current induced by the temperature gradient can be converted into an electric field (an electric current and an electric voltage) by using the inverse spin-Hall effect mentioned above. Namely, by using both the spin-Seebeck effect and the inverse spin-Hall effect, “thermoelectric conversion” in which the temperature gradient is converted into electricity can be achieved.
FIG. 1 shows a structure of a thermoelectric conversion element disclosed in patent literature 1. A thermal spin current conversion section 102 is formed on a sapphire substrate 101. The thermal spin current conversion section 102 has a layered structure in which a Ta film 103, a PdPtMn film 104, and a NiFe film 105 are stacked. The NiFe film 105 has magnetization in an in-plane direction. Further, a Pt film 106 is formed on the NiFe film 105 and the both ends of the Pt film 106 are connected to terminals 107-1 and 107-2, respectively.
In the thermoelectric conversion element having the above-mentioned structure, the NiFe film 105 acts a role of generating the spin-current from the temperature gradient by the spin-Seebeck effect and the Pt film 106 acts a role of generating the electromotive force from the spin-current by the inverse spin-Hall effect. Specifically, when the temperature gradient is applied in the in-plane direction of the NiFe film 105, the spin-current is generated in a direction parallel to the temperature gradient by the spin-Seebeck effect. At this time, the spin-current flows to the Pt film 106 from the NiFe film 105 or the spin-current flows to the NiFe film 105 from the Pt film 106. In the Pt film 106, the electromotive force is generated in a direction that is perpendicular to a spin-current direction and an NiFe magnetization direction by the inverse spin-Hall effect. The electromotive force can be taken out from the terminals 107-1 and 107-2 provided to the both ends of the Pt film 106.
The magnitude of the electromotive force obtained by the above-mentioned thermoelectric conversion element depends on the following three parameters: (1) the magnitude of the spin-current generated in the magnetic film, (2) a spin injection efficiency that is an injection efficiency of the spin-current at an interface between the magnetic film and the metal film, and (3) an electromotive force conversion efficiency that is an efficiency of converting the spin-current into the electromotive force in the metallic film by the inverse spin-Hall effect. Accordingly, in order to realize a spin-current thermoelectric conversion element which can generate a large output power, all three parameters have to be increased.
The following technology is known as the technology to improve the spin injection efficiency among these three parameters.
In non-patent literature 3, a result of investigating the spin-current generated at the interface between the YIG film that is the magnetic film and a Au film that is the metallic film by using a ferromagnetic resonance (FMR) method is shown. According to the result, when a YIG/Au/Fe/Au structure in which the Au film with a thickness smaller than a spin diffusion length (35 nm) is sandwiched between the YIG film and the Fe thin film is used, the large spin-current can be obtained.
In non-patent literature 4, a YIG/Au/Fe/Au multilayered structure is shown. When this multilayered structure is produced, the interface is cleaned by an Ar ion sputter. By this process, the spin-current increases by five times (maximum) in comparison with a case of a YIG/Au structure that is not a multilayered structure.
Non-patent literature 5 discloses the results of the first-principles calculation showing that in case of the YIG/Fe/Ag structure, a spin mixing conductance (a parameter which contributes to the spin injection efficiency) increases by 65% (maximum) according to a magnetic moment density per unit area of the interface between YIG and Ag. By this result, it is suggested that the density of iron atoms at the interface involves increase and decrease of the spin mixing conductance. Further, non-patent literature 4 discloses the experimental result showing that in the YIG/Fe/Au structure, when the thickness of the Fe film is thin and approximately equal to the thickness of one atomic layer, the spin injection efficiency does not change but when it is greater than the thickness of one atomic layer, the spin injection efficiency decreases because it blocks the spin-current.